1. Field of the Invention
The present invention relates to a light-receiving element of an image sensor used for such an image-reading system as a digital camera, an image scanner, a facsimile, a copying machine, etc., as well as a photoelectric conversion device comprising the light-receiving element, specifically to the structure of a light-receiving element suitable for a photoelectric conversion device such as a close contact type image sensor, which is provided with a comparatively large light-receiving element having the opening portion of a pixel with several tens microns or more in size.
2. Related Background Art
In recent years, CCD image sensors, non-CCD image sensors such as CMOS image sensors, etc. have been positively developed as photoelectric conversion devices.
Generally, a photodiode comprising a PN junction of a semiconductor is used for light-receiving elements of these photoelectric conversion devices.
Prior Art (1)
For example, as disclosed by Japanese Patent Application Laid-Open No. 55-154784, there is proposed a structure of a light-receiving element in which the surface of a substrate where no PN junction is formed has a region having the same conductivity type as that of the substrate and a larger impurity concentration than that of the substrate, thereby reducing the dark current to be generated on the surface of the substrate.
FIGS. 29A and 29B show a structure of a conventional light-receiving element. Numeral 201 indicates an n-type semiconductor substrate; 202: a p-type semiconductor layer; 203: an n-type semiconductor layer having an impurity concentration of 5xc3x971015 cmxe2x88x923 to 10xc3x971015 cmxe2x88x923 and a thickness of 0.2 xcexcm to 0.3 xcexcm; 205: a thermal oxide film; 208: an n+ channel stopper; 209: an anti-reflection coating film consisting of nitride; 215 and 216: aluminum electrodes; 228: an n+-type semiconductor layer; and 238: a surface electrode. Symbol DL indicates a depletion layer; and DLS: a surface side portion of the depletion layer.
The anode of a photodiode is formed only with a p-type semiconductor layer 202. When the impurity concentration is lowered, therefore, the property of the ohmic contact with the electrode 215 is degraded. On the contrary, when the impurity concentration is increased, the depletion layer DL is not extended into the semiconductor layer 202.
Prior Art (2)
As disclosed by Japanese Patent Application Laid-Open No. 61-264758, there is proposed a light-receiving element in which the junction capacitance formed by the PN junction is reduced, as the light-receiving element for a one-dimensional photoelectric conversion device.
FIG. 30 shows a top view of a conventional photoelectric conversion device such as a CCD image sensor. Numeral 301 indicates a p-type substrate and numeral 302 indicates an n+-type accumulating portion. A portion surrounded by the n+-type accumulating portion 302 on the p-type substrate 301 is a p-type photoelectric conversion region functioning as a pixel. Symbol PG indicates a photo-gate; SG: a shift-gate; and SR: a CCD shift-register.
In this structure, the PN junction area is reduced, but the PN junction periphery length is increased. It is therefore impossible to sufficiently reduce the capacitance of the PN junction, whereby the sensitivity cannot be increased so much.
Prior Art (3)
For example, as disclosed by Japanese Patent Application Laid-Open No. 1-303752, there is proposed a structure of a photosensitive portion used for a close contact type image sensor, which is intended to reduce the dark current to be caused by the scribe at the end of the chip in the structure of the photosensitive portion.
FIG. 31 shows a cross-sectional view of a light-receiving element of prior art. In FIG. 31, numeral 301 indicates a P-type semiconductor region; 302: an N-type semiconductor region; 303: a P-type shallow channel stop layer; 305: a field oxide film; 306: a P-type substrate; 308: a P-type channel stop layer; 309: an interlayered insulating film; 317 :a light-shielding film for forming an opening portion (OP). A depletion layer (DL) is extended into the P-type semiconductor region 301, whereby electrons of the generated photocarriers (PC) are collected in the N-type semiconductor region 302 by to an internal magnetic field.
Prior Art (4)
For example, as disclosed by Japanese Patent Application Laid-Open No. 64-14958, a photodiode having a cross-sectional structure of N-type substrate/P-type region/N-type region/P-type region is generally used as a light-receiving element of a CCD image sensor.
FIG. 32 shows a cross-sectional view of a light-receiving element of prior art. In FIG. 32, numeral 406 indicates an N-type substrate; 401: a P-type semiconductor region; 402: an N-type semiconductor region; 403: a shallow P-type semiconductor layer; 408: a P+-type channel stop layer; 409: an insulating film; 415: an electrode consisting of polysilicon; and 420: an N-type region of a CCD register.
Prior Art (5)
On the other hand, a photoelectric conversion device employing a light-receiving element is proposed, for example, in Japanese Patent Application Laid-Open No. 9-205588, which uses a photodiode as a light-receiving element and reads the electric charges of the light-receiving element at a time with use of a source follower amplifier by providing this light-receiving element with an electrode and connecting it to the gate electrode of an MOS transistor.
However, when a light-receiving element is employed for an amplifying type photoelectric conversion device which accumulates photo-generated carriers and reads out a signal voltage from a PN photodiode by using charge-voltage conversion means, the sensitivity of the photoelectric conversion device may be degraded in some cases.
For such an amplifying type photoelectric conversion device, the light output is represented by the following formula (1):
Vp=Qp/Csxe2x80x83xe2x80x83(1)
wherein Qp is the quantity of a charge accumulated in the PN photodiode and Cs is a capacitance of the photodiode.
The capacitance Cs of this photodiode can be represented by the formula (2) as shown below, for example, for an amplifying type photoelectric conversion device having a pixel in which an MOS source follower or a reset MOS transistor is connected to a photodiode:
Cs=Cpd+Caxe2x80x83xe2x80x83(2)
Wherein Cpd is a PN junction capacitance of the PN photodiode itself including a light-receiving portion, Ca is the other capacitance of portions connected to the photodiode, and in the above case the other capacitance includes the gate capacitance of the MOS transistor constituting the MOS source follower, the capacitance of the junction between the source and the well of the reset MOS transistor, the overlapping capacitance of the source and the gate, the wiring capacitance, and the like.
Consequently, to realize high sensitivity, it is indispensable that the photo-generated carriers are effectively accumulated and the capacitance of the photodiode for accumulating the carriers is reduced as much as possible.
On the other hand, when light is made incident in the photodiode, electric charges generate in the photodiode and electric charges in and around the depletion layer formed due to a PN junction surface in the semiconductor substrate gather at an anode or a cathode. In this case, when an electrode is attached to this anode or cathode, the electric charges can be taken out as electric signals.
Prior Art (6)
FIG. 33 is a cross-sectional view of a light-receiving element of prior art provided with an electrode. In FIG. 33, numeral 701 indicates a first semiconductor region, and numeral 702 indicates a second semiconductor region to be used as an anode. The conductivity types of those regions are N-type and P-type, respectively. In addition, symbol DL is a depletion layer formed by a PN junction between the first semiconductor region 701 and the second conductor region 702. Although not illustrated in FIG. 33, a reverse bias is applied to between the first semiconductor region 701 and the second semiconductor region 702. In addition, numeral 715 indicates an electrode, which is connected to the second semiconductor region 702 through a contact hole CH of the insulating film 709.
The electrode 715 is composed of a metal, for example, Al as a main component. The electrode 715 is connected to an electrode region formed on the major surface of the semiconductor substrate through a contact hole CH of the insulating film covering the surface of the photodiode. Generally, such light-receiving element is composed by connecting a conductive material such as Al to a semiconductor region so as to obtain photosignals generated by photocarriers photoelectric-converted in the semiconductor region.
For example, when a general RIE (reactive ion etching) method is employed to form this electrode, then over-etching is usually conducted so as to remove unnecessary portions. In this over-etching, however, some ions accelerated by an electrical field pass through the insulating film 709 and reach the major surface of the semiconductor substrate and then damage the vicinity of the interface between the semiconductor and the insulating film, thereby resulting in generation of crystal defects in some cases.
Crystal defects may also generate due to the plasma ashing of a photoresist, etc. even in a step after the electrode is formed, just like in the above case.
In the case of a general light-receiving element, a PN junction exists around a semiconductor region formed on the major surface of the semiconductor substrate to which an electrode is connected and the junction surface reaches the vicinity of the interface between the major surface of the semiconductor substrate and the insulating film in many cases.
Consequently, when an electrode is formed at an inner portion from the junction surface reaching the major surface of the semiconductor substrate, crystal defects due to etching damage generate in the vicinity of the junction surface, and the crystal defects become centers for causing carriers to be generated. The crystal defects generated at a portion of the depletion layer cause a dark current to be generated.
The generated dark current as described above also becomes a factor for causing the dark current to be varied, since the quantity of the crystal defects generated in the vicinity of the interface or the quantity of crystal defects themselves is changed by misalignment of a mask in formation of electrodes, etc. and etching conditions.
The first object of the present invention is to provide a light-receiving element capable of reducing the capacitance of the PN junction of the photodiode portion as much as possible and more effectively utilizing the photo-generated carriers.
The second object of the present invention is to provide a light-receiving element capable of suppressing generation of crystal defects in a semiconductor region where a depletion layer is formed.
(1) The light-receiving element of the present invention comprises:
a first semiconductor region (1, 11, 21, 31, 81) of the first conductivity type;
a second semiconductor region (2, 12, 32, 82) of the second conductivity type, provided on the first semiconductor region;
a third semiconductor region (3, 13, 33, 83) of the first conductivity type, provided between the second semiconductor region and an insulating film;
an electrode region (4, 14, 34, 84) of the second conductivity type, provided in the second semiconductor region where the third semiconductor region is absent on and above the second semiconductor region, and connected to an anode or cathode electrode consisting of a conductor.
Each portion of the light-receiving element may preferably be designed as follows.
The electrode region may be set in a floating state to accumulate photo-generated electric charges; and a bias voltage may be applied to the first semiconductor region so as to apply a reverse bias between the first semiconductor region and the second semiconductor region.
The second semiconductor region provided under the third semiconductor region may be fully depleted, thereby reducing the capacitance.
The electrode region may be shielded from light by the anode or cathode electrode.
A potential slope for moving the photo-generated electric charges towards the electrode region may be formed between the electrode region and the second semiconductor region.
The potential slope for moving the photo-generated electric charges towards the second semiconductor region may be formed between the third semiconductor region and the second semiconductor region and between the first semiconductor region and the second semiconductor region.
The anode or cathode electrode may be connected to the gate of the transistor (M2) of a read circuit.
An internal region (22) of the second conductivity type may be formed inside the second semiconductor region. The internal region (22) has an impurity concentration higher than that of the second semiconductor region and lower than that of the electrode region.
The internal region (22) may consist of a plurality of portions, the portions having an impurity concentration different from each other.
The internal region (22) may be formed so as to enclose the electrode region.
The internal region (22) may be formed so as to be unevenly distributed in an opening portion (OP) formed in a light-shielding film (17).
The internal region (22) may include a region (22A) having a decreased width as the internal region goes away from the electrode region so as to improve the carriers-collecting efficiency.
Each corner of the region (22A) having the decreased width may have an obtuse angle.
The internal region (22) may be extended from the electrode region distributed unevenly in the opening portion formed in the light-shielding film over the center of the opening portion.
The internal region (22) may be formed at a shallower position than the second semiconductor region.
The second semiconductor region may be formed apart from the insulating film for element separation.
The third semiconductor region may be formed apart from the electrode region.
The third semiconductor region may be formed so as to enclose the electrode region.
Each corner of the second semiconductor region may have an obtuse angle.
The electrode region may be provided so as to be distributed unevenly at one end inside the opening portion formed in the light-shielding film, and a contact for applying a voltage to the first semiconductor region therethrough may be provided at the other end inside the opening portion.
A potential slope may be formed in the second semiconductor region from the one end to the other end inside the opening portion.
Each corner of the second semiconductor region may have an obtuse angle, and each corner of the internal region (22) formed in the second semiconductor region may have an obtuse angle.
A doped region (43) having a low impurity concentration may be formed between the third semiconductor region and the electrode region.
An anode or cathode electrode may be formed on or above the doped region.
The anode or cathode electrode may be provided to extend on or above an offset region formed between the third semiconductor region and the electrode region.
The anode or cathode electrode may be provided to extend on or above the interface between the depletion region (DL) formed in the vicinity of the electrode region and the insulating film (9).
The top surface of the second semiconductor region may be covered with the anode or cathode electrode and the third semiconductor region.
The anode or cathode electrode may be connected to the gate of a transistor of the read circuit and the source or drain of a transistor of the reset circuit.
The first semiconductor region may be formed from any one of a semiconductor substrate, an epitaxial layer formed on the semiconductor substrate, and a well formed in the semiconductor substrate.
(2) The light-receiving element of the present invention also comprises:
a first semiconductor region (51, 61, 71, 81) of the first conductivity type;
a second semiconductor region (52, 62, 72, 82) of the second conductivity type, provided on the first semiconductor region;
a third semiconductor region (53, 63, 73, 83) of the first conductivity type, provided between the surface of a semiconductor substrate including the first and second semiconductor regions and an insulating film (9) adjacent to the surface of the semiconductor substrate; and
an anode or cathode electrode (15) consisting of a conductor, the anode or cathode being connected to the second semiconductor region,
wherein the anode or cathode electrode has an extended portion covering an upper part of a portion (59, 69, 89) where the depletion layer (DL) formed between the second semiconductor region and the third semiconductor region is in contact with the insulating film.
Each of the light-receiving element of the present invention may preferably be designed as follows.
The first semiconductor region may be composed of an epitaxial layer, the second semiconductor region may be formed at the top surface inside of the first semiconductor region, and the top surface area of the anode or cathode electrode may be made larger than the top surface area of the second semiconductor region.
The second semiconductor region may be composed of a plurality of portions having an impurity concentration different from each other, and the top surface area of the anode or cathode electrode may be made larger than the top surface area of the second semiconductor region.
The second semiconductor region may be composed of a portion having a high impurity concentration and a portion having a low impurity concentration, and the third semiconductor region may be formed on the top surface of the portion having the low impurity concentration.
The extended portion of the anode or cathode electrode may cover at least a portion on or above the third semiconductor region.
The photoelectric conversion device of the present invention may be obtained by combining those light-receiving elements as described above, a light source such as an LED, and an imaging element.